<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<title>类的基本知识</title>
</head>
<body>
    <button id="btn1">按钮1</button>
    <button id="btn2">按钮2</button>
    <button onclick="demo()">按钮3</button>
    <script type="text/javascript">
        const btn1 = document.getElementById('btn1');
        btn1.addEventListener('click',()=>{
            alert('btn1被点击了');
        })

        const btn2 = document.getElementById('btn2');
        btn2.onclick = function(){
            alert('btn2被点击了');
        }

        const btn3 = document.getElementById('btn3');
        function demo(){
            alert('按钮3被点击了')
        }

    </script>

</body>
</html>